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Suppose there are 4 switches connected using a normal ethernet cables of speed 100mbps as follows:

(S1)---100mbps---->(S2)----100mbps--->(S3)----100mbps--->(S4)

Now suppose the link between S3 and S4 suddenly becomes congested and the transmission rate decreases to 50 mbps. I have a question based on this scenario: Does the path from S1 to S3 need to adjust the transmission rate to deal with the decrease happened on the link between S3 to S4? If yes, how ?

Thank you.

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up vote 3 down vote accepted

Why would congestion on that link cause the transmission rate to drop? I think you are working from an incorrect premise. You can try to run 200 Mbps through a 100 Mbps link, but the transmission rate will stay at 100 Mbps; the excess will just be dropped.

This is layer-2 (switches and ethernet) which don't have any facility for re-transmission of dropped frames. When frames are dropped, they are just lost.

Let's say that S4 is a 48-port switch, and the 48 access ports are each 100 Mbps, as is the uplink port. If all 48 access ports suddenly decided to transmit 100 Mbps each through the 100 Mbps uplink port, the uplink port would transmit 100 Mbps of the 4800 Mbps of traffic trying to use it, and the other 4700 Mbps would be lost.

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Thank you very much for your explanation. So the transmission rate of links is not affected by any factor and it is just representation of the capacity of the link. Can we also say: A link of speed 100Mbps transmits 100mb of data AT TIME, i.e. the link is used for delivery once at time? – Mike Jan 8 at 23:58
    
A data rate is the amount of data transmitted in a given time period. A 100 Mb link means that it can transmit 100 Mb of data every second, also referred to as 100 Mbps. – Ron Maupin Jan 9 at 0:00
    
Can we also say ( A link is used for delivery once at time ) ? I mean it is used by an ethernet card for delivery at a time while the others have to wait – Mike Jan 9 at 0:03
    
I don't understand that statement. You could say that every 1/100,000,000 of a second, the link can transfer one bit, or over one second, the link can transfer 100,000,000 bits. – Ron Maupin Jan 9 at 0:05
    
@Mike, when an interface, uplink or access, transmits information, it will transmit a full layer-2 frame. It will not interleave the bits in the frame with any other frame(s). Each frame is serialized into bits on the wire, but the next frame will not be serialized and mixed in with the first frame, it will have its bits transmitted after the first frame. – Ron Maupin Jan 9 at 0:07

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